Dengue, caused by dengue virus infection, is the most important arthropod-borne viral disease in humans that causes an expanding public health problem in tropical and subtropical regions of the world. There were approximately 50-100 million cases of dengue fever (DF) and 500,000 cases of dengue hemorrhagic fever (DHF) per year, with 2.5 billion people at risk for infection worldwide (Farrar et al., 2007; Halstead, 2007; Normile, 2007). Dengue infection can cause fever, headache and joint pain, leading to mild DHF. It can also cause DHF/dengue shock syndrome (DSS), which is life-threatening (Kalayanarooj et al., 1997).
Dengue virus (DENV) has four genetically and antigenically related viral serotypes, DENV-1, -2, -3, and -4. DENV is positive-sense single-stranded RNA of approximately 11 kb genome of the genus Flavivirus, family Flaviviridae. Flaviviruses encodes a single polyprotein that is processed by host and viral protease to produce three structural proteins, i.e., capsid (C) protein, precursor membrane/membrane (prM/M) and envelope (E) protein, and seven nonstructural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B and NS5) (Rice et al., 1985). The C is an 11 kDa small protein, highly positively charged, that is required for the assembly of nucleocapsid and maturation of viral particles (Kuhn et al., 2002). The NS1 is a 45 kDa glycoprotein that is translocated into the lumen of the ER and secreted from the cell (Schlesinger et al., 1990). It is involved in functions within the viral RNA replication complex (Lindenbach and Rice, 1997, 1999). NS1 protein forms stable oligomers (dimers and hexamers) in solution (Flamand et al., 1999; Winkler et al., 1989). The E protein is a 53 kDa glycoprotein and important for viral entry, its cellular receptor binding capabilities and to the induction of neutralizing antibodies (Kuhn et al., 2002; Pierson et al., 2008; Pokidysheva et al., 2006; Roehrig, 2003). E protein is the external surface of DENY and consists of 90 E protein dimers (Kuhn et al., 2002; Zhang et al., 2003).
The E protein monomer contains three structural and functional domains (Crill and Roehrig, 2001; Modis et al., 2003, 2005; Rey et al., 1995; Roehrig, 2003). E protein domain I (E-DI) is a central n-barrel structure. E protein domain II (E-DII) is organized into two long finger-like structures and contains the flavivirus conserved fusion loop. E protein domain III (E-DIII) adopts an immunoglobulin-like fold and has been suggested to mediate interactions between the virus and the receptor on host cell (Mukhopadhyay et al., 2005). The biological characteristics and epitopes specificity of mouse MAbs have revealed the antigenic structure of flavivirus E. Antibodies that recognize epitopes involving in E-DI are both virus-specific and cross-reactive, predominately non-neutralizing epitopes. Antibodies reactive to E-DII are broadly cross-reactive, but weakly to non-neutralizing. E-DIII elicits serotype-specific, highly protective neutralizing antibodies and cross-reactive antibodies (Crill and Chang, 2004; Crill and Roehrig, 2001; Gromowski et al., 2008; Roehrig et al., 1998; Sukupolyi-Petty et al., 2007).
Antibodies play an important role in protection against DENY. However, antibodies have also been implicated in the development of sever clinical manifestations of DENV infection. Antibody-dependent enhancement (ADE) describes an increase in the efficiency of virus infection in the presence of non-neutralizing or sub-neutralizing concentrations of cross-reactive anti-E immunoglobulins (Halstead and O'Rourke, 1977). The Ab-virus complex attaches to the Fc receptors on circulating monocytes, thereby DENV has been shown to replicate to higher titers in Fc receptor-bearing cells (Halstead, 1988; Littaua et al., 1990). The overall outcomes lead to the potential for more severe disease. Moreover, anti-NS1 antibodies have been reported to confer protection against DENV infection (Falgout et al., 1990; Qu et al., 1993). In addition, it was shown that anti-NS1 antibodies can bind to endothelial cells and cross-react with some self-antigens, induce the expression of cytokines, chemokines, and cause apoptosis (Lin et al., 2005; Lin et al., 2002). These studies suggested that antibodies to E and NS1 proteins are also involved in the pathogenesis of DENV disease. Hence, there is a need for a safe and effective vaccine to DENV.